Gypsum blocks are one of the most economic soil moisture monitoring systems. A gypsum block placed in the soil changes its moisture levels at a similar rate to the soil. The porous gypsum allows water to move in and out of the block as the soil wets (under irrigation or rainfall) and dries (as plants draw water away). This moisture level may be monitored by passing electricity through the block and measuring the resistance.
The gypsum block is a cylindrical block of gypsum into which two electrodes are inserted. Electrical resistance between the electrodes is measured and related to moisture content as a tension.
The tension range measured by the block is highly dependent on soil type. Heavy clays hold onto water particles very tightly and hence a plant may need to exert a large amount of tension on the soil to extract available water. Alternatively, very sandy soils drain freely under gravity and have little capacity to hold onto water.
You will need to select a product that is suitable for your soil type and for the tension range of your crop.
Gypsum is a uniform material and gypsum blocks are manufactured under carefully controlled conditions. As such, gypsum blocks do not need calibration. Readings may be taken manually using a hand-held reader, or with a logger and retrieval device.
Specialist hardware is required to read gypsum blocks. Ask your supplier about the hardware needed to read the block.
The lifetime of a pure gypsum block is variable and is dependent on the soil type and other site-specific factors. Ultimately the gypsum block will dissolve into the surrounding soil, usually over a period of 1-5 years. Composite blocks have a much longer operational lifetime than a pure gypsum block – a 10-year life span is typical.
Factors affecting gypsum block lifetime include:
As with any soil moisture monitoring system it is important to check the conditions in the field at various times. A block nearing the end of its life can give incorrect readings. By checking conditions in the field you will know whether the readings are accurate or not.
Speak to an irrigation consultant or your product distributor to find out how best to meet your requirements. We have developed a range of factsheets to assist you with irrigation management. See the related links below.